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Was on another no heat call today. Beckett AFG burner and Weil McLain boiler and a honeywell single zone aquastat relay. Customer said the circuit breaker to the furnace had tripped. Checked out the aquastat and I had power at L1 and L2 coming in and the contactor would close when a call for heat was initiated. Turns out the 'stat was burned out on the neutral side on both the circulator and burner circuits so I replaced it and all is well. My question has to do with some basic circuitry. When I was testing the 'stat with my meter, I had one lead on the return pipe(ground) and I checked the hot on both the burner and circulator and I got 120V on each, as well as 120V at the primary control (both the incoming from the aquastat and the outgoing to the blower/transformer) When I moved the lead I had on the return pipe to the L2 neutral on the stat, I also had 120V but when I put it on the neutrals on the burner and circulator circuits I got nothing. So, thats when I decided the neutrals were shorted out and sure enough there was a big black mark on the back of the board. How come I still was getting 120V on the burner/circ. circuits when I had the ground on the return pipe? I'm thinking I understand the problem, but I'd like an explanation from someone who has been doing this longer than I have!